Jill of all trades, but mistress of few !

[Julie]

Hi Robin,

Thanks for the quick reply. I guess my continuing to set ambitions above abilities is an on-going victory for hope over experience!

I'm interested in what you say about servo motors, rather than steppers. If you servos at all, I guess it's best to use them on all axes to have common control logic?

I appreciate your point about a 4ft gantry. I had seen the Blacktoe 4 x 8 project on buildyourcnc.com, which uses MDF for construction. However, I did note that the gantry structure was made pretty beefy. As for keeping it running square, at that size I'm guessing you need to provide a motor on each side to drive the x-axis?

Still a lot to learn.

Julie

[HiltonSteve]

Welcome to the forum Julie, promise I won't swear (too much)!

I have been considering designing a larger machine also and have been lurking around youtube and various 'other' DIY CNC sites for idea's before I put mouse to screen.

Found this site on my travels - http://www.automatedwoodworks.com/

I think this guy has done a half decent job, take a look at his gallery and his youtube video's. He is using steppers which are a much cheaper option than servo's and he is getting decent rapid speeds.

Like other people have said, with steppers its a trade off between speed and accuracy, on my current machine I am using 16 x 5 ballscrews which are directly driven off of a 23 size 3.1Nm stepper, with a 5A driver I can run comfortably at 5m/min and I have had it up to 7m/min. I am running at 1/4 step so this give 800 steps per 5mm, this = 0.0063mm which is fairly accurate!

I am thinking of using the same steppers and drivers for a larger machine but gearing them 2:1, this should double the rapid speeds and half the accuracy but still give me 0.0126mm per micro step. Pretty sure that the 23 steppers will be fine but may consider going up a size to be on the safe side.

Will probably belt drive it and just use one stepper for the long X axis with a shaft to link the drive belts on either side to avoid any crabbing.

Just my two penneth worth!

[Robin Hewitt]

I'm guessing you need to provide a motor on each side to drive the x-axis?

Hi Julie

That's to be avoided like the plague :nope:

First you have to decide what kind of accuracy you want to achieve for the cut. Think resolution. The motor moves the table by one increment. A small increment improves the accuracy of the cut. A large increment improves your top speed which is handy when you want to move 8' ASAP.

When you have an accuracy figure in mind you can decide how to drive it. Screws, rack and pinion, belting? The lower the accuracy the cheaper it becomes. It's all trade offs, what are you trying to achieve? Are we talking mass production where speed is king? Are we talking bespoke and it doesn't really matter if it takes 2 minutes to move end to end at a lowly 2 cm/second?

Robin

[Gary]

I think she means the Y axis.
Moving a large gantry is better to drive from both sides or you can get crabbing even with good quality profile rails.
This is more promenent if you are cutting on one side.

Hi Julie

That's to be avoided like the plague :nope:

First you have to decide what kind of accuracy you want to achieve for the cut. Think resolution. The motor moves the table by one increment. A small increment improves the accuracy of the cut. A large increment improves your top speed which is handy when you want to move 8' ASAP.

When you have an accuracy figure in mind you can decide how to drive it. Screws, rack and pinion, belting? The lower the accuracy the cheaper it becomes. It's all trade offs, what are you trying to achieve? Are we talking mass production where speed is king? Are we talking bespoke and it doesn't really matter if it takes 2 minutes to move end to end at a lowly 2 cm/second?

Robin

[Philmollatt]

Hi all
Just an idea regarding the x axis 2 motor thing, if i were doing it i would put a lead screw/ballscrew on each side conected by a timing belt and pulleys, with a double pulley on one side then drive the second pulley from the motor, also this gives you some flexibility with pulley sizes, small motor pulley for accuraccy large motor pulley for speed. Shoud also work with rack and pinion systems with a long drive shaft.
Thats the way i would go if i were building it from scratch, that way its just the one motor driving both sides of the axis, and a swap out pulley set up with the progam,
Thanks for listening
Still a newbie so dont take me too seriously..

[Smiler]

Hi Julie,

RE: driving the long axis with 2 motors. You don't have to with R&P!

All I did was to make a shaft with a pinion on both ends and a drive pulley inboard of one of them. Then simply mount the motor in line with the pulley and you are driving both sides of the gantry with 1 motor, no crabbing, simples.

If you want to get fancy you can use a zero backlash pinion on one end but I didn't think it needed on a router.

Jeff.

[Gary]

The principle is the same if using screw or rack.
drive it from both sides and you won’t have a problem.

[Smiler]

The principle is the same if using screw or rack.
drive it from both sides and you won’t have a problem.

Of course it is. However, once you get into long travel lengths, unless you need the super duper accuracy of ballscrew then IMO R&P wins hands down. It is also much easier/mechanically simpler to drive both sides of a rack with a single motor and a single motor is what I would propose. Maybe Techserve could give us some insight on how they do things on long axes.

End of the day this is a router and R&P will give Julie all the accuracy she needs whether she is carving long panels or cutting parts for furniture. All I would suggest is she drives both sides with a SINGLE motor (bought from Zzap).

Jeff.